A prior art-pumped fiber laser consists of a double cladding optical fiber pumped by high power diode lasers and a doped core. The cladding has a numerical aperture ranging, for example, from approximately 0.45 to 0.6. The pump power from a fiber coupled high power diode laser may be coupled to the cladding or fiber core in some instances via a dichroic mirror as FIG. 1 and FIG. 2 illustrate. Pump light is directed by the multimode fiber 100. A dichroic filter 110 is inserted between the multimode pump fiber and the double cladding fiber 140. The dichroic filter 110 is transparent to the pump light 101 and acts as a broad-band reflection filter for the signal beam 120. Alternatively, the dichroic filter 210 reflects the pump beam and is transparent to the signal beam 220. Several publications and patents describe methods to use dichroic filters for end pumping fibers e.g. U.S. Pat. Nos. 5,917,648, 5,966,391 and “Experimental studies on narrow-linewidth Yb3+-doped double-clad fiber-laser cavities based on double-clad fiber Bragg gratings” MICROWAVE AND OPTICAL TECHNOLOGY LETTERS 44 (1): 53-56 Jan. 5, 2005; and “Double-clad fibers enable lasers to handle high power” Laser Focus World, January, 1999. Dichroic filters provide both the high reflector (broadband spectrally, several nm) for the signal beam and the isolation between the pump beam and signal beam.
A variation without dichroic filter is to use Fiber Bragg grating in the core of the pumped fiber for the purpose of providing a narrow bandwidth reflection filters (sub nm) at the signal wavelength as described by “Nd-doped double-clad fiber amplifier at 1.06 μm” JOURNAL OF LIGHTWAVE TECHNOLOGY 16 (4): 562-566 April 1998. For multimode doped fiber core, Fiber Bragg gratings do not exist. It is desirable to have a narrow bandwidth spectral output coupler to yield a high spectral brightness laser.
Volume holographic gratings have been shown to be an extremely accurate and temperature-stable means of filtering a narrow passband of light from a broadband spectrum. This technology has been demonstrated in high power density applications (e.g. “Improvement of the spatial beam quality of laser sources with an intracavity Bragg grating”, Optics Letters, 28,4, 2003; and “Wavelength Stabilization and spectrum narrowing of high Power multimode laser diodes and arrays by use of Volume Bragg Gratings”, Optics Letters, 29, 16, 2004). such as stabilization of high power diode lasers with narrow full-width-at-half-maximum (FWHM) passbands as small as 0.2 nm with several kW/cm2 intensity levels.
Photorefractive materials, such as LiNbO3 crystals and certain types of polymers and glasses, have been shown to be effective media for storing multiple volume holographic gratings such as for optical filters or holographic optical memories with high diffraction efficiency and storage density (“Angle And Space Multiplexed Holographic Storage Using The 90-Degrees Geometry”, Opt Commun 117, (1-2),1995; “Cross-Talk For Angle-Multiplexed And Wavelength-Multiplexed Image Plane Holograms” Opt Lett 19 (21), 1994; “Folded shift multiplexing”, OPT LETT 28 (11),2003; and “Holographic multiplexing in photorefractive polymers ”, OPT COMMUN 185, 2000).